1. Field of the Invention
The present invention generally relates to pulse-width modulation circuits and pulse-width modulation methods, and more particularly to circuits and methods related to pulse-width modulation of a self-oscillation type.
2. Description of the Related Art
A class-D amplifier is used in electronic products, such as digital versatile disc (DVD) players, because of its superior performance compared to an analog amplifier. Pulse-width modulation (PWM) has become the core of class-D amplifiers.
Conventional PWM circuits generate a pulse of a certain width by comparing an audio signal with a triangular wave or a sawtooth wave, and so the conventional class-D amplifier requires a ramp signal generator to modulate a pulse's width. The ramp signal generator generates a ramp signal between 200 kHz to 500 kHz, and if the ramp signal generator is incorporated into an integrated circuit, the ramp signal generator occupies a large area on the chip. Therefore, a PWM circuit that generates a PWM signal without the ramp signal generator, would reduced the area required on a chip for PWM.
FIG. 1 illustrates a class-D audio amplifier with a self-oscillation type PMW circuit as disclosed in U.S. Pat. No. 4,531,096. FIG. 2 is a timing diagram illustrating waveforms of signals at parts of the conventional class-D audio amplifier shown in FIG. 1.
Referring to FIG. 1 an integrator of the class-D audio amplifier integrates an input signal 10a and a feedback signal V0. The integrator includes an operational amplifier 11, capacitors 12a and 12b, and resistors 13 and 21. The feedback signal, (i.e. a PWM signal V0) output from the PWM circuit, is applied to an inverting input terminal of the operational amplifier 11 through a resistor 18 and the input signal 10a is applied to a non-inverting input terminal of the operational amplifier 11. Referring to FIGS. 1 & 2, an output voltage V1 of the integrator (an output of the operational amplifier 11) has a form of a triangular wave, and an output voltage V2 (output by a digital inverter 14a) has the form of a pulse wave. Additionally, an output signal V5 of an ON-OFF-timing control circuit 20 and an output signal V6 of a pulse amplifier 33 both have the form of a pulse wave having a dead time. The PWM signal V0 is an output signal of a switching circuit including a p-channel metal oxide semiconductor (PMOS) transistor 22a and an n-channel metal oxide semiconductor (NMOS) transistor 22b. The duty factor of the PWM signal V0 varies linearly with a voltage variation of the input signal 10a. The PWM signal V0 is demodulated by a low-pass filter 34, and is provided to a speaker 35. Therefore, the conventional PWM circuit of the class-D audio amplifier generates the PWM signal having a duty factor, which may be varied according to the variation of the input signal.